Droop control has commonly been used with distributed generators for relating their terminal parameters with power generation. The generators have also been assumed to have enough capacities for supplying the required power. This is however not always true, especially with renewable sources with no or insufficient storage for cushioning climatic changes. In addition, most droop-controlled literatures have assumed a single dc-ac inverter with its input dc source fixed. Front-end dc-dc converter added to a two-stage photovoltaic (PV) system has therefore usually been ignored. To address these unresolved issues, an improved droop scheme for a two-stage PV system has been developed in the paper. The developed scheme uses the same control structure in both grid-connected and islanded modes, which together with properly tuned synchronizers, allows mode transfer to be seamlessly triggered. Moreover, the proposed scheme adapts well with internal PV and external grid fluctuations, and is hence more precise with its tracking, as compared with the traditional droop scheme. Simulation and experimental results have verified these expectations, and hence the effectiveness of the proposed scheme.
- Photovoltaic (PV) power systems
- Power distribution control
- Power electronics
- Droop control